Elimination of implant damage during manufacture of HBT

ABSTRACT

During the conventional manufacture of HBTs, implant damage occurs which leads to enhanced internal base diffusion. This problem has been overcome by making the base and base contact area from a single, uniformly doped layer of silicon-germanium. Instead of an ion implant step to selectively reduce the resistance of this layer away from the base, a layer of polysilicon is selectively deposited (using selective epi deposition) onto only that part. Additionally, the performance of the polysilicon emitter is enhanced by means a brief thermal anneal that drives a small amount of opposite doping type silicon into the SiGe base layer.

FIELD OF THE INVENTION

[0001] The invention relates to the general field of heterojunction bipolar transistors (HBT) with particular reference to providing good base contact without introducing ion implant damage.

BACKGROUND OF THE INVENTION

[0002]FIG. 1 is a schematic cross-sectional view of a heterojunction bipolar transistor (HBT) of the prior art. N+ polysilicon emitter 10 sits atop base layer 11 a of silicon-germanium. The actual area where it makes contact to the Si-Ge (i.e. the heterojunction) is restricted by the presence of insulating layers 18 and 19 of silicon nitride and silicon oxide respectively, the former having been used as a hard mask during manufacturing. The emitter pedestal is further protected by means of spacers 17 on its vertical sidewalls.

[0003] In order to be able to contact the base region, SiGe layer 11 a has been laterally extended from both sides as layer 11 b. The resistivity of 11 a needs to be relatively high (to ensure efficient injection of electrons from the emitter) while the resistivity of 11 b needs to be as low as possible in order to minimize the base resistance (which in turn controls the cutoff frequency of the device). During manufacturing, 11 a and 11 b are deposited as a single layer of relatively high resistivity. After formation of the emitter pedestal, the latter acts as a mask during implantation of acceptor ions (usually boron) into 11 b whereby its resistivity is then substantially lowered.

[0004] Other features seen in FIG. 1 include N type silicon body 16 from whose upper surface N+collector volume 12 extends downwards. Connection to collector 12 is made through N+sinker 14 which is connected to 12 through buried N+ layer 13. Electrical isolation from other parts of the integrated circuit is achieved by means of dielectric-filled deep and shallow trenches 15.

[0005] There are several problems associated with the design seen in FIG. 1. In particular, when external base 11 b is doped by ion implantation, implant damage occurs which leads to enhanced internal base diffusion. This, in turn, will enhance emitter junction diffusion, leading to an increase in emitter and base junction width which will degrade the speed of the device.

[0006] The present invention teaches how the device can be manufactured without the need to include the damaging ion implant step that is inherent to the prior art. This allows achievement of very shallow base and emitter junctions for the same thermal budget.

[0007] A routine search of the prior art was performed with the following references of interest being found:

[0008] In U.S. Pat. No. 6,169,007 B1, Pinter shows a HTB with an epi base. In U.S. Pat. No. 5,846,867, Gomi et al. disclose a method for a Si-Ge base in a HBT. Cho et al., in U.S. Pat. No. 5,897,359, show a method for a HBT while Kim shows a process for a transistor having an epi base in U.S. Pat. No. 6,060,365.

SUMMARY OF THE INVENTION

[0009] It has been an object of at least one embodiment of the present invention to provide a heterojunction bipolar transistor that is free of implant damage at the emitter-base interface (which leads to enhanced internal base diffusion).

[0010] Another object of at least one embodiment of the present invention has been that said transistor not be subject to enhanced emitter junction diffusion which degrades the speed of the device.

[0011] Still another object of at least one embodiment of the present invention has been to provide a process for manufacturing said transistor.

[0012] These objects have been achieved by making the base and base contact area from a single, uniformly doped layer of silicon-germanium. Instead of an ion implant step to selectively reduce the resistance of this layer away from the base, a layer of Second doping type polysilicon is selectively deposited (using selective epi deposition) onto only that part. Additionally the performance of the polysilicon emitter is enhanced by means a brief thermal anneal that drives a small amount of N-type dopant into the SiGe base layer.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013]FIG. 1 shows a HBT of the prior art.

[0014]FIG. 2 is the starting point for the process of the present invention.

[0015]FIG. 3 is the structure after deposition of the SiGe layer.

[0016]FIG. 4 is the above structure after deposition of a polysilicon layer.

[0017]FIG. 5 shows said polysilicon layer after it has been patterned to form an emitter.

[0018]FIG. 6 shows said emitter after addition of insulating spacers on its sidewalls.

[0019]FIG. 7 illustrates selective epitaxial deposition of doped P-silicon onto SiGe.

[0020]FIG. 8 shows the base contact area after patterning.

[0021]FIG. 9 illustrates how base width is reduced by means of a brief heat treatment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0022] We will describe the present invention through a detailed description of a process for its manufacture. This will also serve to explain the structure of the present invention. Referring now to FIG. 2, the process of the present invention begins with the provision of N-type silicon body 16 in which is present collector volume 12 that extends downwards from the top surface. Collector 12 is located between dielectric-filled shallow isolation trenches 15 a and 15 b. Collector contacting area 14 is located between trench 15 b and a third shallow isolation trench 15 c that is placed on the far side of 14.

[0023] The first step, as illustrated in FIG. 2, is the deposition of silicon oxide and nitride layers 19 and 18 respectively onto the surface of silicon body 16. These layers are then patterned to become a hard mask that defines opening 22, located so as to overlap shallow isolation trenches 15 a and 15 b while protecting trench 15 c.

[0024] Next, as seen in FIG. 3, layer of silicon-germanium 31 is deposited onto the surface of 16 so that opening 22 is overfilled and silicon nitride layer 18 is covered. Layer 31 is uniformly doped (typically with boron) as part of its deposition process. Layer 31 contains between about 8 and 20 atomic % germanium, has a resistivity between about 3,000 and 4,000 ohm cm., and is deposited to a thickness between about 200 and 1,000 Angstroms.

[0025] Now, moving on to FIG. 4, layer 42 of silicon oxide is deposited onto silicon-germanium layer 31 and then patterned to form opening 45 that lies over and underlaps collector area 12. This is followed by the deposition of N-type polysilicon layer 41 which then fully covers all exposed surfaces, including filling of opening 45. Layer of polysilicon 41 is deposited to a thickness between about 1,500 and 3,000 Angstroms and has a resistivity between about 20 and 60 ohm cm. After layer 41 has been planarized, layer 43 of silicon oxide is deposited thereon.

[0026] Referring next to FIG. 5, layer 45 of silicon oxide is now patterned and used as a hard mask for the formation of emitter pedestal 51. Then, as shown in FIG. 6, with the remains of layer 42 being left in place, insulating spacers 62 were formed on pedestal 51's sidewalls, resulting in the complete encapsulation of the emitter in insulation, except for opening 45.

[0027] Referring now to FIG. 7, in a key step, layer of doped P-silicon 71 is deposited onto silicon-germanium layer 31, and only onto layer 31. This was accomplished by using selective epitaxial deposition, using low pressure chemical vapor deposition (LPCVD) with a Si₂H₆ gas source. Layer of doped P-silicon 71 was deposited to a thickness between about 2,000 and 2,500 Angstroms and has a sheet resistance between about 10 and 40 ohms per square.

[0028] Then, as shown in FIG. 8, layers 31 and 71 were patterned to form the base contact area. Finally, in another key step, the structure is subjected to a rapid thermal anneal—typically, heating to a temperature between about 1,000 and 1,100° C. for between about 10 and 20 seconds. This causes N-dopant material from polysilicon emitter 51 to diffuse into silicon-germanium layer 31, thereby forming region 91, as shown in FIG. 9. Region 91 is an extension of the emitter and it serves to control the base width as well as to ensure that, at the interface to the base, the emitter comprises single crystal material. Region 91 is typically between about 150 and 200 Angstroms thick. 

What is claimed is:
 1. A process to form a base contact for a heterojunction bipolar transistor, comprising: providing a first doping type silicon body having a surface that includes a collector area; depositing a layer of uniformly doped silicon-germanium onto said surface; forming a first doping type polysilicon emitter pedestal, having insulating spacers on its sidewalls, over, and in contact with, said collector area; by means of selective epitaxial deposition, depositing a layer of oppositely doped silicon onto only said layer of silicon-germanium; patterning said oppositely doped silicon and silicon-germanium layers to form said base contact; and by means of a rapid thermal anneal, causing first type-dopant material from said polysilicon emitter to diffuse a distance into said silicon-germanium layer.
 2. The process described in claim 1 wherein said layer of silicon-germanium contains between about 8 and 20 atomic % germanium.
 3. The process described in claim 1 wherein said layer of silicon-germanium has a resistivity between about 3,000 and 4,000 ohm cm.
 4. The process described in claim 1 wherein said layer of silicon-germanium is deposited to a thickness between about 200 and 1,000 Angstroms.
 5. The process described in claim 1 wherein said layer of opposite type doped silicon is deposited to a thickness between about 1,500 and 3,000 Angstroms.
 6. The process described in claim 1 wherein said layer of opposite type doped silicon has a sheet resistance between about 20 and 60 ohms per square.
 7. The process described in claim 1 wherein said distance that first type-dopant material diffuses from said polysilicon emitter into said silicon-germanium layer during said rapid thermal anneal is between about 150 and 200 Angstroms.
 8. The process described in claim 1 wherein the step of depositing a layer of doped silicon onto only said layer of silicon-germanium, by means of selective epitaxial deposition, further comprises use of LPCVD with a Si₂H₆ gas source.
 9. The process described in claim 1 wherein said rapid thermal anneal further comprises heating to a temperature between about 1,000 and 1,100° C. for between about 10 and 20 seconds.
 10. A process to manufacture a heterojunction bipolar transistor, comprising: providing an first doping type silicon body having a first surface that includes an active collector area between first and second shallow isolation trenches and a collector contacting area between a third shallow isolation trench and said second trench; depositing a first layer of silicon nitride onto said first surface and patterning said silicon nitride so as to provide a first opening that overlaps said first and second shallow isolation trenches; depositing a layer of silicon-germanium onto said first surface, in said first opening, and onto said first layer of silicon nitride; depositing a first layer of silicon oxide on said layer of silicon-germanium and then patterning said first silicon oxide layer to form therein a second opening that overlies and underlaps said active collector area depositing a layer of first doping type polysilicon to fully cover said first silicon oxide and said silicon-germanium layers, including filling said second opening; planarizing said layer of first doping type polysilicon and then depositing thereon a second layer of silicon oxide; using said second layer of silicon oxide as a hard mask, patterning said polysilicon layer to form an emitter pedestal having vertical sidewalls; forming insulating spacers on said sidewalls; by means of selective epitaxial deposition, depositing a layer of opposite type doped silicon onto only said layer of silicon-germanium; patterning said opposite type doped silicon and silicon-germanium layers to form a base pattern region; and by means of a rapid thermal anneal, causing N-dopant material from said polysilicon emitter to diffuse a distance into said silicon-germanium layer.
 11. The process described in claim 10 wherein said layer of silicon-germanium contains between about 8 and 20 atomic % germanium.
 12. The process described in claim 10 wherein said layer of silicon-germanium has a resistivity between about 3,000 and 4,000 ohm cm.
 13. The process described in claim 10 wherein said layer of silicon-germanium is deposited to a thickness between about 200 and 1,000 Angstroms.
 14. The process described in claim 10 wherein said layer of opposite type doped silicon is deposited to a thickness between about 1,500 and 3,000 Angstroms.
 15. The process described in claim 10 wherein said layer of uniformly doped opposite type silicon has a sheet resistance between about 20 and 60 ohms per square.
 16. The process described in claim 10 wherein said distance that first dopant type material diffuses from said polysilicon emitter into said silicon-germanium layer during said rapid thermal anneal is between about 150 and 200 Angstroms.
 17. The process described in claim 10 wherein said layer of polysilicon is deposited to a thickness between about 2,000 and 2,500 Angstroms.
 18. The process described in claim 10 wherein said layer of polysilicon has a resistivity between about 10 and 40 ohm cm.
 19. The process described in claim 10 wherein the step of depositing a layer of doped silicon onto only said layer of silicon-germanium, by means of selective epitaxial deposition, further comprises use of LPCVD with a Si₂H₆ gas source.
 20. The process described in claim 10 wherein said rapid thermal anneal further comprises heating to a temperature between about 1,000 and 1,100° C. for between about 10 and 20 seconds.
 21. A base contact for a heterojunction bipolar transistor, comprising: a first doping type silicon body having a surface that includes a collector area; a layer of uniformly doped silicon-germanium, on said surface, that overlaps and contacts said collector area, thereby defining said base contact; over said silicon-germanium layer, a polysilicon emitter pedestal that overlaps said collector area and that touches said base contact in an area that underlaps said collector area; a layer of opposite type doped silicon on, and only on, said layer of silicon-germanium wherever said silicon germanium is not covered by said emitter pedestal; and an amount of first dopant type material that has diffused from said polysilicon emitter into said silicon-germanium layer for a distance.
 22. The base contact described in claim 21 wherein said layer of silicon-germanium contains between about 8 and 20 atomic % germanium.
 23. The base contact described in claim 21 wherein said layer of silicon-germanium has a resistivity between about 3,000 and 4,000 ohm cm.
 24. The base contact described in claim 21 wherein said layer of silicon-germanium has a thickness between about 200 and 1,000 Angstroms.
 25. The base contact described in claim 21 wherein said layer of opposite type doped silicon has a thickness between about 1,500 and 3,000 Angstroms.
 26. The base contact described in claim 21 wherein said layer of opposite type doped silicon has a sheet resistance between about 20 and 60 ohms per square.
 27. The base contact described in claim 21 wherein said distance that first dopant type material has diffused from said polysilicon emitter into said silicon-germanium layer is between about 150 and 200 Angstroms.
 28. A heterojunction bipolar transistor, comprising: a first doping type silicon body having a first surface that includes an active collector area between first and second shallow dielectric isolation trenches and a collector contacting area between a third shallow dielectric isolation trench and said second trench; a first layer of silicon nitride on said first surface, patterned so as to provide a first opening that overlaps said first and second shallow isolation trenches; a layer of uniformly doped silicon-germanium on said first surface within said first opening and on said first layer of silicon nitride, said layer of silicon-germanium having the form a base contact area; on said silicon-germanium layer, a polysilicon emitter pedestal that overlaps said active collector area and underlaps said base contact area, said emitter pedestal being encapsulated in a layer of insulation, except for a second opening in said insulation at its interface with said silicon-germanium layer where polysilicon directly contacts the silicon-germanium, said second opening underlapping said active collector area; a layer of doped opposite type doped silicon on, and only on, said layer of silicon-germanium; and an amount of first dopant type material that has diffused from said polysilicon emitter into said silicon-germanium layer for a distance.
 29. The transistor described in claim 28 wherein said layer of silicon-germanium contains between about 8 and 20 atomic % germanium.
 30. The transistor described in claim 28 wherein said layer of silicon-germanium has a resistivity between about 3,000 and 4,000 ohm cm.
 31. The transistor described in claim 28 wherein said layer of silicon-germanium has a thickness between about 200 and 1,000 Angstroms.
 32. The transistor described in claim 28 wherein said layer of opposite type doped silicon has a thickness between about 1,500 and 3,000 Angstroms.
 33. The transistor described in claim 28 wherein said layer of opposite type doped silicon has a sheet resistance between about 20 and 60 ohms per square.
 34. The transistor described in claim 28 wherein said distance that first dopant type material has diffused from said polysilicon emitter into said silicon-germanium layer is between about 150 and 200 Angstroms.
 35. The transistor described in claim 28 wherein said polysilicon emitter has a resistivity between about 10 and 40 ohm cm. 